Completely decouple the Stud.IP frontend from API calls to Opencast

[tgloeggl]

Is your feature request related to a problem? Please describe.
There are parts in the plugin, where users have to wait for calls to Opencast to be finished before the user can continue to work in Stud.IP. While this might be acceptable during the configuration of the plugin on the admin settings page, it is not when users are opening the videos tab in a course and alike.
This problem is even worse with high timeouts when calling the Opencast API, since the UI than even hangs longer. On the other hand are these high timeouts necessary to make sure calls to Opencast succeeds because they seem to work erratic sometimes.

Describe the solution you'd like
Users should always have the most speedy UI as possible. If calls to Opencast need to happen, they should always happen in the background and the user merely informed by the progress and when its done.

Describe alternatives you've considered
We already have decoupled lots of opencast calls and deferred them to cronjobs, but there a still things remaining which need to be addressed, f.e. but not only:

• Setting world wide visibility for videos
• Creation of series for users and courses

Additional context

• See also Wenn das Opencastsystem nicht reagiert, hängt in Stud.IP die ganze Veranstaltung #1021, where it was tried to tackle the problem on the "My courses"-page
• The timeout for Opencast calls needs to be increased to 60 seconds, even before the suggested changes are done